Maritime transport accounts for about 90% of world trade and emissions from large ships are responsible for a major fraction of the undesirable gases discharged into the atmosphere. For this reason, the European Union, as part of their RD&I programmes, have given a high priority to the reduction of CO2 emissions in the maritime industry.
The high fuel consumption of ships is a consequence of the resistance, or drag force, that they must overcome. That drag increases power requirements and therefore cost and emissions also. Part of the resistance stems from the wave-making drag and for this reason hull designs always attempt to minimise drag, although most possible improvements have already been considered and it is unlikely that significant additional reductions will be achieved through this route.
Another major element of the resistance encountered by a ship comes from the hull-water interaction. Viscous effects cause frictional drag that opposes movement, as well as an added mass that moves with the ship and contributes considerable inertia. The added mass is typically 1/4 to 1/3 of that of the ship, hence any reductions have important implications.
As mentioned in a recent post on biomimetics, nature frequently offers evolved solutions capable of improving current industrial practices. For example, inspired by the superhydrophobic surfaces found in many plants and animals (lotus leaves, cuticles of certain insects, butterfly wings, etc.) biomimetic materials have been developed for coating the surface of a ship’s hull at the point of hull-water interaction.
It might seem intuitive that a smoother surface would reduce drag on the viscous layer on the wet surface. But there is another lesson we can learn from nature, by considering the skin of certain fishes that are excellent swimmers, for example, many species of shark. Their skin is not smooth at all, but instead consists of small scales or denticles, with a complex geometry of riblets and channels that vary in size and orientation across the shark’s body. The effect is to reduce viscous resistance by controlling the characteristics of the flow on the microscopic scale.
It is not exactly clear how the denticles create this drag reduction and up to now attempts to reproduce the effect have only achieved reductions of less than 10%.
Incidentally, there are some precedents: golf balls ceased to be smooth long before we understood why.
Until we get a good grasp of the physical phenomena involved, it will not be easy to mimic biological structures with a view to improving hull coatings and reducing drag. But simulation allows us to analyse the different geometries and orientations of the denticles, studying velocities, pressures and vorticity near the boundary layer. It is expected that this will lead to advances in the design of bioinspired coatings or finishes, which can be created by additive manufacturing and tested using scale models. In a not too distant future, new coatings may be contributing effectively to the reduction of viscous drag in ships.
Juan Carlos Suárez is Director of the Center for Research in Structural Materials (CIME) of the UPM
Álvaro Rodríguez Ortiz is a researcher at CIME and at ETSI Navales de la UPM
